206 SYMMETRY OF THE SPOROPHYTE 
bring it about has led Goebel to the conclusion that unequal illumination 
is a determining cause; for he found that in Dzphysccum the flattening 
of the unilateral sporogonia always takes place on the illuminated side 
(Fig. 104). The primary advantage which is gained by the dorsiventral 
development is the enlargement of the assimilating system. Haberlandt 
has shown how considerable the assimilatory activity is in the capsule 
of Mosses, and has specially pointed out in the case of Buxbaumia how 
much more extensive, as well as better stocked with chloroplasts, the 
enlarged face of the capsule is, than is the side directed downwards. 
A secondary advantage is that the oblique position 
is effective in connection with the scattering of 
the spores. 
Such facts relating to the Bryophyta clearly 
indicate that the radial type of construction is 
the fundamental one for their sporogonia. Not 
{only are the departures from that type relatively 
few, and far from being extreme examples as 
h compared with dorsiventrality elsewhere, but also 
they may in some cases at least be put in 
definite relation with external causes, and the 
altered form be shown to have a favourable 
/ 
if 
biological effect. When to this it is added that 
the dorsiventrality appears comparatively late in 
the individual development, the case seems fully 
Fic. 104. made out for the priority of the radial construc- 
Diphyscium foliosum. Longi- tion of the sporogonium of Bryophytes. 
tudinal section of a stem bearing 
S 
‘\ 
a sporogonium. The arrow indi- The infinitely greater variety of form among 
cates the prevalent incidence of ss 
light. (After Goebel.) the Vascular Plants in some measure confuses 
the “question of a fundamental type of symmetry 
for them. Moreover, the issue is further obscured by the diversity of 
their embryogeny: so long as the initial characters of their embryos are 
held accurately to reflect their evolutionary story, this difficulty will 
remain, but in a previous chapter this doctrine has been held open 
to doubt. In the present discussion of the symmetry of the shoot in 
Vascular Plants their embryology will be put temporarily aside, and it 
will be considered towards the close of this chapter. Questions of 
symmetry in Vascular Plants are also complicated by the presence of a 
foliar development. This difficulty will weigh most with those who 
entertain some phytonic theory of the shoot; but into their difficulties 
we need not, enter, since reasons have been given for not sharing their 
view (Chapter XI.). Assuming, in accordance with our earlier discussions, 
a strobiloid theory, the shoot will be habitually regarded as an entity, 
and its symmetry as a whole will be held to be determined by the 
equal or unequal development of the appendages, with or without a 
corresponding development of the axis which bears them. 
